Various kinds of structures have been proposed for rotors for electric motors. In one example, a rotor for a multilayered flux-barrier-type reluctance motor includes a plurality of slits arranged inside a core to form a plurality of magnetic paths. Permanent magnets are inserted in the slits. A structure of such a motor of the multilayered flux-barrier-type reluctance motor is described below.
FIG. 6 is an example cross-section of a rotor structure in a conventional multilayered flux-barrier-type reluctance motor. A rotor 1 of FIG. 6 is made of silicon steel plates stacked on top of each other. Each silicon steel plate includes a plurality of slits 2 arranged therein and magnetic paths 4 are formed among the slits 2. Specifically, the rotor 1 includes a plurality of poles (e.g., four poles are illustrated in the drawing) formed in a circumferential direction. Each pole has a plurality of (e.g., five in the drawing) slits 2. The slits 2 for each pole are arranged in parallel with each other in a radial direction. Each slit 2 has a substantially circular-arc shape or U-shape that opens outward. A permanent magnet 3 is partially inserted into the slits 2. Specifically, the permanent magnet 3 is inserted into a portion of the slits 2 away from the outer periphery of the slits 2 and closer to the inner periphery of the slits 2. Magnetic poles of the permanent magnet 3 are directed perpendicular to the magnetic paths to allow cancellation of magnetic flux leakage.
A stator 5 is disposed outside the rotor 1. As is well known in the art, the stator 5 includes slots 6 each having a winding in which electric current flows to generate torque in the electric motor. The rotor 1 includes an output shaft 7 which is disposed in the center part of the rotor 1 to externally transmit torque generated in the electric motor.
In the rotor 1 including the permanent magnets 3 inserted in the slits 2, magnetic flux flows as illustrated in FIG. 7 which is a partial enlarged view of the rotor 1 of FIG. 6. In FIG. 7, a torque current for generating torque is supplied to the winding located at position Iq in the stator. The torque current constantly flows in the same direction in the motor. In FIG. 7, the torque current flows in the winding at Iq from the front side toward the back side of the drawing. Further, a field current flows in the windings at positions Id1 and Id2 to form magnet flux along the magnetic paths 4. In FIG. 7, the current is made to flow from the back side toward the front side of the drawing in the winding at Id1 and vice versa in the winding at Id2 to generate anticlockwise rotary torque to the rotor 1. Accordingly, the magnetic flux is formed around the windings at Id1 and Id2 in a direction indicated by arrows shaded by hatched lines in the drawing around the positions of the windings in accordance with the right-hand screw rule. At the winding at Iq, a clockwise force is generated in accordance with Fleming's left-hand rule. Thus, the rotary torque is generated in an anticlockwise direction in the rotor 1 in accordance with the law of action and reaction.
Next, the direction of magnetic poles of the permanent magnet 3 inserted into the slits 2 is described. The torque current constantly flows in the same direction at the position Iq, as mentioned above. That is, the torque current flows from the front side toward the back side of the drawing. The magnetic flux is also formed around the winding at Iq in accordance with the right-hand screw rule. The direction of the magnetic flux is perpendicular to the slits 2, generating a large magnetic resistance and therefore, impeding generation of the magnetic flux. In practice, magnetic flux even smaller than the field magnetic flux is formed in this direction. Such a magnetic flux is called magnetic flux leakage. In FIG. 7, the magnetic flux leakage interlinks with the winding from the inside to the outside at Id1 and vice versa at Id2, generating rotary torque in the clockwise direction in the rotor 1 in accordance with Fleming's left-hand rule. This direction, however, cancels the torque generated by the torque current, decreasing torque generation of the rotor. This is called armature reaction. To decrease the armature reaction, permanent magnets 3 are inserted in a direction to cancel the magnetic flux leakage, as illustrated in FIG. 7. Specifically, the permanent magnet 3 inserted into the left-hand slits has the south pole on the internal end and the north pole on the external end relative to the radial direction of the rotor 1, while the permanent magnet 3 inserted into the right-hand slits has the north pole on the external end and the south pole on the internal end.
The permanent magnets 3 thus inserted in the above-described directions need to have magnetomotive force large enough to cancel the magnetic flux leakage. Specifically, the magnitude of the torque current increases/decreases depending on the instruction of torque to be generated by the electric motor. The magnetomotive force of the permanent magnets 3 is determined to be equivalent to the magnetic flux leakage which would be generated when the torque current having an assumedly maximum value is supplied. If the permanent magnets 3 having a larger magnetomotive force are inserted, the armature reaction would rather increase depending on the direction of rotation.
Further, the magnetic flux output from each permanent magnet 3 enters neighboring magnets 3 inside the rotor 1 in the radial direction, as illustrated in FIG. 8, so that the magnetic flux of the permanent magnet 3 does not reach the surface of the rotor 1 to increase the field magnetic flux. This is equivalent to an ordinary synchronous reluctance motor, which is realized by removing the permanent magnets 3 from FIG. 6, in that the strength of the magnetic poles formed in the rotor 1 is determined in accordance with the intensity of the field current and that the magnitude of torque generated in the electric motor depends on the strength of the magnetic poles and the magnitude of the torque current. A power factor increases because the magnetic flux leakage is smaller than that of the ordinary reluctance motor.
However, the fact that the magnetic power of the permanent magnets is not used for the torque of the electric motor, although the permanent magnets are inserted into the rotor, is very inefficient from a viewpoint of energy efficiency. The description therefore discloses a rotor capable of increasing torque generated by an electric motor using magnetic force of permanent magnets inserted into the rotor.